1. Field of Invention
This invention relates to a ribbon driving mechanism for a typewriter capable of positioning either a printing ribbon or a correction tape selectively at a printing position by moving a holder holding the printing ribbon and correction tape.
2. Description of the Prior Art
European Laid Open Publication No. 38,215 (corresponding to application SN No. 81301649.0), for example, discloses a typewriter of the foregoing type. This conventional typewriter is provided with a control member having a first cam for positioning a printing ribbon held on a holder at a printing position, a second cam for positioning a correction tape held on the holder, at the printing position, and a control cam for feeding the printing ribbon, and connected to a reversible motor. A printing ribbon lifting mechanism, a correction tape lifting mechanism and a printing ribbon feeding mechanism, of the typewriter, are adapted to be controlled according to the respective shapes of the corresponding cams on the basis of the rotation of the control member by way of the reversible motor. During ordinary printing operation, the printing ribbon is fed by a predetermined feed after the printing ribbon has been positioned at the printing position.
In the above apparatus, a single cam member is provided with a first and second cam on one side thereof, and a control cam is provided on the other side thereof. Accordingly, the cam member needs to repeat a cyclic motion through a predetermined angle during printing operation, in order to retain the holder at the lifted position, as well as to feed the printing ribbon. Thus, the printing speed is limited by the intrinsic character of the mechanism and furthermore, problems are likely to occur due to the complicated position detecting mechanism required for detecting the rotational position of the motor and the motor control circuit.